1. Field of the Invention
This invention relates to a method and apparatus for improving one or more soil characteristics during vacuum excavation of the soil, thereby improving the replaceability of the soil upon completion of the project necessitating excavation of the soil. More particularly, this invention relates to a method and apparatus for beneficiating soil during one or more of the steps employed in vacuum excavation of the soil.
2. Description of Related Art
In order to install or access already-installed underground facilities, such as pipelines and the like, it is often necessary to excavate in pavements, sidewalks and other locations. Vacuum excavation as a means for the removal of dirt, water or other material from an excavation site is well known. Vacuum excavation is widely considered to be superior to traditional excavation techniques such as manual or mechanical digging, primarily due to the reduced risk of damage to underground facilities that may be present at the excavation site. Vacuum excavation typically involves the use of a vacuum system to pull a slurry or debris stream that may exist or that may be created by the use of mediums capable of loosening hard-packed material. For example, loosening of the hard-packed material may be achieved through the use of air lances or water jets, which inject air or water under high-pressure into the hard-packed material. The use of air lances and water jets for loosening the soil virtually precludes the possibility of damage to the underground pipelines or other buried utilities. Important applications for vacuum excavation systems include environmental cleanup and the collection of slurry created in the process of directional drilling. Vacuum excavation is particularly useful in small excavations such as the “potholing” or “keyholing” of utilities. Potholing and keyholing refers to the excavation of a hole to access or repair utilities. Potholing and keyholing are preferred over prior art techniques such as digging shovels, backhoes, and the like which often result in damage to the utilities and the corresponding demand for expensive repair and reconstruction. In addition to the increased costs associated with damage repair and increased labor costs associated with digging, such damage may present danger to workers or others in the immediate vicinity of the excavation. When vacuum excavation is utilized for potholing and keyholing, access is possible without risk of damage to the utilities. In addition, such small excavations provide the benefit of working from the surface.
Exemplary of the prior art existing with respect to the technique of vacuum excavation are U.S. Pat. No. 5,016,717 to Simmons et al., which teaches a vehicle mounted vacuum excavation system having a tank mounted on a lift mechanism to facilitate the raising and tilting of the tank to release excavated materials by dumping through an openable rear hatch or end wall of the tank in which a vibrator is used to prevent compaction of particles within the tank to facilitate release when the tank is tilted for dumping; U.S. Pat. No. 5,295,317 to Perrott which discloses a mobile vacuum excavation system and tank featuring a slurry water recovery system to reuse excavation water; and U.S. Pat. No. 6,604,304 B1 to Slabach et al., which teaches a vacuum excavation system utilizing pneumatic or hydraulic agitation of the holding tank to suspend vacuum-excavated materials for release, thereby facilitating the efficient emptying of the tank.
Backfill of an excavation and restoration of the pavement or sidewalk structures upon completion of the job for which the excavation was carried out ideally includes the replacement of suitable excavated soil, compacted to engineering specifications or to match pre-existing density. However, the soil may not be suitable for such purpose due, for example, to high clay content, and/or the soil may not be amenable to compaction due to excess or inadequate moisture and/or high clay content. In such cases, stabilization and/or beneficiation of the soil is required to render the soil more suitable for such purposes. If it is not stabilized or beneficiated, the soil must be hauled away and replaced with imported select material, adding significant dollar and environmental costs to the restoration.
Traditional mechanical methods of stabilizing soils are best suited for large-scale work in which it is practicable to set up and operate on-site the large equipment needed. Alternatively, centralized plants can be operated, but this requires transporting unstabilized materials from the point of excavation to the plant and transporting stabilized materials back to the excavation. Small excavations typically are poorly served by such methods. On the other hand, manual methods for stabilizing are often impractical, for example in heavy clays, and are labor-intensive, slow and have limited productivity.
The use of soil beneficiating agents that operate by means of one or a combination of direct chemical reaction, ion exchange, cementing action, dilution effects, gradation changes and the like may make the excavated soil suitable for backfill. For example, hydrated lime through ion exchange has long been used as a beneficiating agent for clays, and Portland cement has long been used to provide additional strength through cementing action. As another example, dry sand has been used to reduce overall moisture content and improve overall gradation when mixed with wet fine-grained soil. In all cases, intimate and thorough mixing are important, and this is assisted by breaking the soil into small pieces, thereby enlarging the amount of surface area available for beneficiation and reducing the distance the beneficiating agent has to travel to the interior for chemical and other effects to occur. Traditionally, mixing of the beneficiating agent with the original soil is usually performed after excavation of the soil in mechanical devices such as pug mills, and sometimes by hand with shovels. When large areas are involved, disc harrows or similar mechanical methods are employed. However, each of these traditional means requires the performance of a separate step and additional equipment.